Audience Composition is the age and gender demographics of the audience for a particular advertising medium.
For Example: For a particular website, if total number of visitors is 20,000. The audience composition for the website will be something like:
• 16,000 are female and 4,000 are male
• 75% of people are aged between 25-45 and 10% are aged between 45-50 and 15% are aged between 18-25
Some inferences can be drawn here like the site attracts female viewership that are in child bearing age. Advertisements targeting such audience can then be placed for these websites.